In 1899, F. Robertshaw invented a thermostat gas control system for home hot water heaters. With the invention of the thermocouple and its use in operating electromagnetic fuel control valves, such as is shown in Mantz, U.S. Pat. No. 2,351,277, the safety of these gas control systems was enhanced. However, no major conceptual improvements in low cost valve controls have occurred since that time. Thermocouple/thermopile controls produce very low voltage and are unable to provide the power in the form of potential and current (particularly voltage) required to operate modern semiconductors or other simple electronic circuits used to control combustion appliances. Thermocouples have a relatively slow response time. Additionally, it would be useful to be able to detect carbon monoxide for controlling the combustion device because hazardous concentrations of carbon monoxide are often produced before any other malfunction can be detected.
In many countries, unvented gas heaters are not approved for home use without an added safety device such as a safety shutoff valve actuated by an oxygen depletion sensor (ODS). The ODS determines when low levels of oxygen occur and shuts off the combustion source. When the oxygen concentration decreases, the unstable pilot flame jumps off the pilot orifice, causing the thermocouple to cool. Another type of thermocouple control, classified as an oxygen depletion sensor, is described in Great Britain Patent No. 992,102 to Societe Gama. The oxygen depletion sensor suffers the same deficiencies as the other thermocouple controls. None of the thermocouple controls can sense the presence of carbon monoxide, which often reach dangerous levels before significant depletion of oxygen. The ODS suffers from premature shutoff and inability to detect deadly CO.
Other control systems typically require external power sources or provide complex circuitry for accomplishing control. Such systems include flame rectifiers, photocell systems, spectroscopic analyzers, and oxygen sensors. These systems will now be discussed.
Flame rectifiers such as that shown in Smith et al., U.S. Pat. No. 2,748,846, have been used for obtaining faster response to flame-out (loss of flame), but these systems are expensive, require external power and often have slower response times. Serber, U.S. Pat. No. 4,405,299, also shows such a device.
Smith et al. also teaches the use of a lead sulfide photoconductive cell 13 for sensing flame emitted from a burner. Bogdanowski et al., U.S. Pat. No. 2,835,886, shows the use of a photocell 24 in conjunction with an external power source for indicating the decrease in concentration of oxygen in an area surrounding the flame. Other devices using photocells in combination with external power sources are shown in Westbrook, U.S. Pat. Nos. 2,898,981, Pounds; 3,086,147, Giuffrida; 3,238,423; Sellors, Jr., 3,576,556; and Guilitz, 4,059,385.
Miller, U.S. Pat. No. 3,102,257, shows a device utilizing a filter for eliminating all visible light, except that of wavelengths absorbed by carbon monoxide or other gas which absorbs visible light having the wavelength of the transmitted light. The photocell used for detecting the particular band passed by the filter is used in combination with an external power source.
Other control systems for modern gas appliances utilize flame color monitors for monitoring the gas flame. In the late 1960's, Briggs, U.S. Pat. No. 3,301,308, and Alexander et al., U.S. Pat. No. 3,304,989, provided a safety control for portable heaters and like equipment with a fuel feed control system responsive to the color of a flame. Both of these systems use cadmium sulfide cells to increase combustion appliance safety. These systems are complex and expensive, requiring external power sources, but are still unreliable.
In United Kingdom Patent No. 2,052,725, an oxygen sensor is utilized to control burning efficiency through the monitoring of the oxygen concentration of the burner exhaust gases. The oxygen concentration is used to regulate the air-fuel ratio. The control is a complex device requiring outside power, is expensive, is not fail-safe, and is ineffective in controlling combustion when hazardous amounts of carbon monoxide are present.
Carbon monoxide (CO) is often present as a byproduct of combustion. It can accumulate to harmful levels when gas appliances or other combustion devices malfunction or are used without adequate ventilation. The risks due to the presence of CO have increased in recent years due to energy conservation measures, which reduce air exchange, or substitute zone heating for central heating.
A low-cost CO sensor would greatly increase the safety of gas heaters. The use of unvented gas appliances, such as ranges and clothes dryers, is also hazardous because of CO production and would benefit from the use of a CO sensor. However, most instruments presently used for detection of CO are not suitable for widespread use, such as on gas appliances and heaters, or in portable instruments for the home, auto or workplace.
Several devices for measuring carbon monoxide or carbon dioxide are described below.
Yant et al., U.S. Pat. No. 2,531,592, teaches a device for detecting carbon monoxide or other gases through use of a catalyst coated on a thermopile. Yant et al. suffers from the same defects as do the devices utilizing thermocouples or thermopiles for otherwise controlling combustion devices.
Klug, U.S. Pat. Nos. 2,549,974 and 2,561,802, and Farr et al., U.S. Pat. No. 2,553,179, use the photocharacteristic change of a substance to detect carbon monoxide. The device uses a complex electronic bridge circuit, along with the National Bureau of Standards colorimetric indication gel invented by Martin Shepherd, as a detector. However, the photocharacteristic change of the indication gel is reversible only by the flushing of the gel with a particular regenerating gas. As a result, ambient carbon monoxide would eventually trigger an alarm due to buildup of CO over time, requiring that the indicator be changed periodically to prevent false alarms due to accumulated CO. Furthermore, the device requires an external power source. Such control systems are large and expensive and not suitable for gas appliances or other mass market applications.
Gafford et al., U.S. Pat. No. 3,114,610, teaches a device for continuous gas analysis by measuring the change in pH caused by carbon dioxide as an indirect measure of carbon monoxide. Transmission of light from an external source to a photocell is changed due to the color change of a sensing gel containing a pH-sensitive dye. The change in the amount of light transmitted is detected with the photocell. Gafford et al. also requires an external power source and is subject to interference from smog and other gases.
Guenther, U.S. Pat. No. 3,754,867, teaches a chemical system which is reversibly absorbent for carbon dioxide, and includes a pH color-changing dye and a photocell. This system uses an outside power source for supplying power to the light source for producing a signal. The system would also be subject to nuisance shutoff and unreliability due to ubiquitous carbon dioxide. The applicability of the device for measuring sulfur dioxide and other gaseous acidic anhydrides is mentioned in Guenther.
There is a need for a control for combustion devices which is compact, does not require external power sources, and which is inexpensive to manufacture and use. There is also a need for more efficient controls for such devices than exist with thermocouple controls and similar devices having slow response times. Furthermore, it is desirable to provide a control which operates with a quantum device having an abrupt cutoff, rather than linearly or gradually as a thermocouple does. In this regard, it would be desirable to provide a spectral source to aid in the detection of toxic or volatile gases. The present invention overcomes the technological and economic disadvantages of previous devices, and offers a safe, efficient, convenient and self-sufficient control for combustion devices.